The present invention generally relates to image processing apparatuses, and more particularly to an image processing apparatus which corrects an error diffusion process that subjects an image data to a pseudo halftone processing.
Conventional printers are designed by putting weight on particularly the reproducibility of characters. However, recently, there are more opportunities to process images on a personal computer or the like, and it has also become desirable to print images such as natural images and graphics on the printer with a satisfactory reproducibility.
In the printer which is designed by putting weight on the reproducibility of the characters, the size of one dot is set larger than one dot pitch. For example, the diameter of the dot is set within a range of 1.4 to 2.0 times one dot pitch. By such a setting, the contrast and the visibility of the characters are improved. In other words, the reproducibility of the characters and line drawings is improved by setting the diameter of the dot to 1.8 times one dot pitch, for example.
On the other hand, in the case of an image such as the natural image and graphics, the image is generally printed according to the halftoning method. Various halftoning methods have been proposed, and the error diffusion method is regarded as a promising method. According to the error diffusion method, a difference between the input image density and the output image density is distributed with respect to the neighborhood pixels. As a result, the density of the image as a whole is maintained, and the resolution can be maintained to a considerably high state. For this reason, even if the error diffusion method is applied to a printer having a low resolution, the error diffusion method can print images having a satisfactory quality compared to the case where other halftoning methods are used.
However, if the size of one dot is set larger than one dot pitch when printing the natural image or the like, each dot is printed with an area which is larger than the original dot area. Consequently, the density of the printed image as a whole becomes high, and there was a problem in that saturation occurs for densities greater than or equal to an intermediate level. In addition, this problem was particularly notable when the printing is carried out using the error diffusion method.